A piezoelectric actuator driven by a share mode system has heretofore been known as the piezoelectric actuator. The share mode system refers to a driving system which causes displacement by a driving electric field in a direction crossing a polarized field direction of a piezoelectric body at right angles. One example of the piezoelectric actuator using such a driving system is shown in FIG. 16.
A piezoelectric actuator 71 shown in FIG. 16 is used in an ink jet head, a plurality of piezoelectric bodies which are comb teeth 76 or driving portions 74 are formed in the form of comb teeth 76 on a substrate 72, and slits 75 between the comb teeth 76 are closed by a lid plate 77 to form cells 73 having substantially rectangular parallelepiped shapes. When openings in comb teeth tips of the piezoelectric actuator 71 are closed by a nozzle plate 9 including nozzles 8, an ink jet head 70 using the cells 73 as ink chambers is obtained. Moreover, when a driving electric field is applied in a direction parallel to the polarized field direction of the comb teeth 76 which are formed of piezoelectric materials to constitute the driving portions 74, the comb teeth 76 are deformed, volumes of the cells 73 change, and ink stored in the cells 73 can be discharged.
The piezoelectric actuator 71 is prepared in the steps illustratively shown in FIGS. 17(a) to 17(e). First, a piezoelectric material 86 is prepared (FIG. 17(a)), and fired (shrunk to a certain extent as shown in FIG. 17(b)). Then, the polarization treatment is performed (FIG. 17(c)). In FIG. 17(d), fine slits are formed by working processing using a dicing saw or the like, and the driving portions 74 displaced by the driving electric field are sandwiched between the plurality of slits 75 is arranged in the form of the comb teeth to form electrodes 88 on the wall surfaces in the slits 75 as is shown in FIG. 17(e). Said slits are spaces in which the ink is stored. Thereafter, as shown in FIG. 16, the lid plate 77 made of a glass plate or the like is attached, and the openings in the comb teeth tips are covered with the nozzle plate 9 including the nozzles 8 to form the cells 73 to be charged with the ink.
Moreover, there is disclosed another type of an ink jet (recording) head of a piezoelectric type in Patent Document 1 (See Japanese Patent No. 2873287).
The ink jet (recording) head disclosed in Patent Document 1 is characterized in that ink channels (corresponding to the cells) and dummy channels are alternately formed being separated by side walls of piezoelectric bodies, electrodes formed in the respective ink channels are formed as a common electrode, electrodes formed in the respective dummy channels are used as individual electrodes, and the ink is prevented from contacting a passivation film formed on the individual electrode. An electric field is applied to the electrodes formed in the respective channels to change volumes in the ink channels, and ink droplets are discharged. According to Patent Document 1, by the above-described characteristics, the electric field does not act on the ink during the driving, and any electrolysis of the ink by a low electric insulation withstand voltage does not occur regardless of a defect or non-defect of the passivation film. Therefore, a life of the head is prolonged, there is not any possibility that physical properties of ink are changed during the driving the head, and it is therefore described that the actuator largely contributes a high printing quality.
Furthermore, the present inventors have proposed a piezoelectric actuator, which is different from the piezoelectric actuator shown in FIG. 16, in Patent Document 2 (See WO 02/070265 pamphlet).
The actuator disclosed in Patent Document 2 is a comb teeth type piezoelectric actuator, characterized in that a plurality of piezoelectric actuators comprising a plurality of piezoelectric layers are aligned/arranged in the form of the comb teeth, and the side surface of each comb tooth comprises an as-fired surface. As described in Patent Document 2, because of the above-described characteristics, the surface of the comb teeth type piezoelectric actuator is not easily contaminated, a short circuit defect or the like caused by sticking matters during the practical use does not easily occur, and the actuator is superior in durability and reliability.
However, the actuators disclosed in the above-mentioned Patent Documents have problems to be improved.
First, the conventional piezoelectric actuator 71 shown in FIG. 16 has a problem caused by the driving in the share mode system in which the driving electric field is generated in the direction crossing the polarized field direction at right angles. That is, since the actuator cannot be driven at such high electric field intensity that a polarization of the actuator is changed, a large electric field induced strain amount is not obtained. If a high driving electric field intensity is applied, the polarization of the actuator gradually changes during the driving, and eventually a drop of the strain amount is caused. Therefore, it has been difficult to obtain a large displacement in the actuator as long as the share mode system is used.
Moreover, during the manufacturing, since the electric field direction for polarization is different from an electric field direction for driving, the electrodes for use in the polarization treatment should be formed, and removed after the polarization treatment. This means that the formation of electrodes for driving is required thereafter. Accordingly, the number of forming steps increases, and the workloads become larger.
Additionally, in the ink jet (recording) head described in Patent Document 1, since the common and individual electrodes are formed on the side walls of the piezoelectric body, it is laborious to form the electrodes on the narrow spaces of the side walls with plating, sputtering or the like in order to cope with a demand for high densification of the ink jet (recording) head in recent years. This would often cause the lowering of the conducting reliability. When a conductive liquid is used as an ink, it is indispensable to form the passivation film in order to insulate the electrodes, and extra forming steps have been required. Furthermore, depending on the driving polarity, it is necessary to form the passivation film in order to prevent the electrolysis of the ink regardless of whether or not the ink has conductivity, and it has been difficult to reduce the number of steps required for the manufacturing.
For the comb teeth type piezoelectric actuator disclosed in Patent Document 2 by the present applicant, since it is not necessary to form the electrodes on the side walls by the plating, sputtering or the like, the conducting reliability of the electrode does not drop even in the arrangement in a high density. However, it is indispensable to form the passivation film in order to insulate the electrodes, and the extra forming steps have been required.